Showing papers by "Pusan National University published in 1996"

Aqueous polyurethane dispersions

Abstract: This review describes basic chemistry, preparation process, and physical properties of aqueous polyurethane disperisons and the derived films, along with the methods of post treatment to modify the properties. Basic way to render a polyurethane water dispersible without external emulsifier has been described. Regarding the methods of preparation, four major processes are described and compared. Methods to improve the relatively poor water and solvent resistance of aqueous polyurethane dispersions which is introduced by the hydrophilicity and linear structure of polyurethane have been discussed with an emphasis on acrylate incorporations.

A senescence-associated gene of arabidopsis thaliana is distinctively regulated during natural and artificially induced leaf senescence

Abstract: We have characterized the structure and expression of a senescence-associated gene (sen1) of Arabidopsis thaliana. The protein-coding region of the gene consists of 5 exons encoding 182 amino acids. The encoded peptide shows noticeable similarity to the bacterial sulfide dehydrogenase and 81% identity to the peptide encoded by the radish din1 gene. The 5′-upstream region contains sequence motifs resembling the heat-shock- and ABA-responsive elements and the TCA motif conserved among stress-inducible genes. Examination of the expression patterns of the sen1 gene under various senescing conditions along with measurements of photochemical efficiency and of chlorophyll content revealed that the sen1 gene expression is associated with Arabidopsis leaf senescence. During the normal growth phase, the gene is strongly induced in leaves at 25 days after germination when inflorescence stems are 2–3 cm high, and then the mRNA level is maintained at a comparable level in naturally senescing leaves. In addition, dark-induced senescence of detached leaves or of leaves in planta resulted in a high-level induction of the gene. Expression of the sen1 gene was also strongly induced in leaves subjected to senescence by 0.1 mM abscisic acid or 1 mM ethephon treatment. The induced expression of the gene by dark treatment was not significantly repressed by treatment with 0.1 mM cytokinin or 50 mM CaCl2 which delayed loss of chlorophyll but not that of photochemical efficiency.

Crystallinity and morphology of segmented polyurethanes with different soft-segment length

Abstract: A series of polycaprolactone/4,4′-diphenylmethane diisocyanate/1,4-butanediol (PCL/MDI/BDO) segmented polyurethanes of different compositions was synthesized by solution polymerization. The molecular weight of PCL diols used was in the range of 1600–7000. The crystallinity and morphology of these polymers were studied by using DSC, dynamic mechanical analysis, WAXD, and polarizing microscopy methods. It was found that the crystallinity of PCL prepolymers was depressed in segmented polyurethanes. A lower limit of PCL molecular weight was found, below which the PCL segments were not able to crystallize at the usual processing conditions. This limit of molecular weight is in the range of 2000–3000 and exhibits a slight increase with increasing hard-segment content of polyurethanes. The glass transition temperature related to the PCL segment regions in polyurethane specimens deviated from that of pure amorphous PCL prepolymer to a higher temperature. The deviation resulted from the crystallization of PCL segments and also the influence of hard segments. The formation of hard-segment domains becomes very difficult for polyurethanes having low hard-segment content and short hard-segment length. There is a lower limit of hard-segment content and segment length. Only above that limit do the polyurethanes have enough hard-segment domains acting as physical crosslinks at temperatures above the melting point of the PCL crystals. The structural characteristics of segmented polyurethanes which may exhibit a shape memory effect are also discussed. © 1996 John Wiley & Sons, Inc.

Waterborne polyurethanes and their properties

Abstract: Anionomer-type waterborne polyurethanes (PUs) were obtained from poly(β-methyl-δ-valerolactone) glycol (PMVL) and isophorone diisocyanate, following a prepolymer mixing process. The soft-hard segment phase separation in response to the variations of composition and structure of PU has been studied from the dynamic mechanical measurements of the emulsion cast films. The structural variation included ionic and hard segment content, molecular weight of NCO-terminated prepolymer, and type and length of the soft segment. It was found that phase separation is more sensitive to the soft segment length, rather than the soft segment content. With only phase separation, the rubbery modulus was significant even with lower hard segment content. Phase separation was much more pronounced with PU from poly(tetramethylene adipate) glycol, rather than from PMVL and poly(caprolactone) © 1996 John Wiley & Sons, Inc.

Cluster accretion shocks as possible acceleration sites for ultra-high-energy protons below the greisen cutoff

Abstract: Three-dimensional hydrodynamic simulations of large scale structure in the Universe have shown that accretion shocks form during the gravitational collapse of one-dimensional caustics, and that clusters of galaxies formed at intersections of the caustics are surrounded by these accretion shocks. Estimated speed and curvature radius of the shocks are 1000-3000 \kms and about 5 Mpc, respectively, in the $\Omega=1$ CDM universe. Assuming that energetic protons are accelerated by these accretion shocks via the first-order Fermi process and modeling particle transport around the shocks through Bohm diffusion, we suggest that protons can be accelerated up to the {\it Greisen cutoff energy} near $6\times 10^{19}$ eV, provided the mean magnetic field strength in the region around the shocks is at least of order a microgauss. We have also estimated the proton flux at earth from the Virgo cluster. Assuming a few (1-10) \% of the ram pressure of the infalling matter would be transferred to the cosmic-rays, the estimated flux for $ E \sim 10^{19}$eV is consistent with observations, so that such clusters could be plausible sources of the UHE CRs.

Anti-Invasive Activity of Ursolic Acid Correlates with the Reduced Expression of Matrix Metalloproteinase-9 (MMP-9) in HT1080 Human Fibrosarcoma Cells

Abstract: We examined the anti-invasive activity of ursolic acid (UA) on the highly metastatic HT1080 human fibrosarcoma cell line. UA reduced tumor cell invasion through a reconstituted basement membrane in a transwell chamber. A significant down-regulation of matrix metalloproteinase-9 [MMP-9; Mr 92,000 gelatinase/type IV collagenase (gelatinase B)] by UA was detected by Northern blot analysis. However, MMP-2 [Mr 72,000 gelatinase/type IV collagenase (gelatinase A)] and membrane-type MMP were constantly expressed, and the expression of tissue inhibitor of metalloproteinase (TIMP)-1 and TIMP-2 also was not changed after 3 and 6 days of treatment with UA. Quantitative gelatin-based zymography confirmed a markedly reduced expression of MMP-9 but not MMP-2 after treatment with UA. To confirm the UA-induced down-regulation of MMP-9 expression, we constructed a secreted alkaline phosphatase (SEAP) reporter vector including MMP-9 promoter. After transfection of MMP-9/SEAP reporter vector into HT1080 cells, reduced SEAP activity was detected after treatment with UA. These results suggest that down-regulation of MMP-9 contributes to the anti-invasive activity of UA in HT1080 cells.

Contributions to the cosmic ray flux above the ankle: clusters of galaxies

Abstract: Assuming that particles can be accelerated to high energies via diffusive shock acceleration process at the accretion shocks formed by the infalling flow toward the clusters of galaxies, we have calculated the expected spectrum of high-energy protons from the cosmological ensemble of the cluster accretion shocks. The model with Jokipii diffusion limit could explain the observed cosmic ray spectrum near with reasonable parameters and models if about of the infalling kinetic energy can be injected into the intergalactic space as the high energy particles.

Pharmacological coupling and functional role for CGRP receptors in the vasodilation of rat pial arterioles.

Abstract: In this study, we investigated the signal transduction underlying the vasodilator action of calcitonin gene-related peptide (CGRP) in the rat pial arterioles. In an in vivo experiment, changes in p...

Two anhydrous zeolite x crystal structures, cd46si100al92o384 and cd24.5tl43si100al92o384

Abstract: The structures of fully dehydrated, fully Cd2+-exchanged zeolite X, Cd46Si100Al92O384 (Cd46-X; a = 24.935(8) A), and that of fully dehydrated Cd2+- and Tl+-exchanged zeolite X, Cd24.5Tl43Si100Al92O384 (Cd24.5Tl43-X; a = 24.858(9) A), have been determined by single-crystal X-ray diffraction methods in the cubic space group Fd3 at 21(1) °C. Cd46-X was prepared by ion exchange in a flowing stream of 0.05 M aqueous Cd(NO3)2 for 2 days. Cd24.5Tl43-X was prepared similarly using a solution 0.025 M each in Cd(NO3)2 and TlNO3. Each crystal was then dehydrated at 450 °C and 2 × 10-6 Torr for 2 days. Their structures were refined to the final error indices R1 = 0.055 and R2 = 0.077 with 544 reflections for Cd46-X, and R1 = 0.054 and R2 = 0.051 with 272 reflections for Cd24.5Tl43-X; I > 3σ(I). In the structure of dehydrated Cd46-X, Cd2+ ions are located at two different crystallographic sites. Sixteen Cd2+ ions fill site I, at the centers of the double six-rings; each Cd2+ ion is octahedrally coordinated by framewo...

Effectiveness of Green Tea Tannin on Rats with Chronic Renal Failure

Abstract: The effects of green tea tannin on nephrectomized rats were examined. There were increases in blood urea nitrogen, serum creatinine, and urinary protein, and a decrease in creatinine clearance in the nephrectomized control rats, whereas better results for these parameters were obtained in rats given green tea tannin after nephrectomy, demonstrating a suppressed progression of the renal failure. When the renal parenchyma was partially resected, the remnant kidney showed a decrease in the activity of radical scavenger enzymes. Green tea tannin, however, was found to lighten the kidney under such oxidative stress. Mesangial proliferation and glomerular sclerotic lesions, which were conspicuous in the rats that were not given green tea tannin after nephrectomy, were also relieved.

Role of intracellular calcium as a priming signal for the induction of nitric oxide synthesis in murine peritoneal macrophages

Abstract: Because the role of intracellular Ca2+ in the two-signal process for the induction of nitric oxide (NO) synthesis is controversial, this study was undertaken to examine the role of Ca2+ in the transcriptional regulation of inducible NO synthase (iNOS) in murine peritoneal macrophages. Treatment of the cells with thapsigargin (TG) or 2,5-di-(t-butyl)-1,4-benzodihydroquinone (tBuBHQ), which are the specific and potent Ca(2+)-ATPase inhibitors of endoplasmic reticulum (ER), showed modest effects on tumoricidal function, whereas TG or tBuBHQ in combination with interferon-gamma (IFN-gamma) or lipopolysaccharide (LPS) showed marked effects on tumoricidal function of the cells. The tumoricidal effects of the activated macrophages were correlated with the amount of NO synthesis, and totally abrogated by the use of NOS inhibitor, NG-monomethyl-L-arginine (NGMMA). The increases in NO synthesis was reflected as increased amounts of iNOS mRNA by Northern blotting. To confirm that iNOS induction was due to the changes in the intracellular Ca2+ level, the acetoxymethyl ester of 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA-AM), an intracellular Ca2+ chelator, was used. Blocking the increase of cytosolic free Ca2+ significantly decreased the induction of NO synthesis. To demonstrate that intracellular Ca2+ acts as a 'priming' signal rather than a 'triggering' signal on the induction of NO synthesis by murine peritoneal macrophages, we designed several experiments. When the cells were treated with TG 6 hr after the treatment with IFN-gamma, there was no increase in NO synthesis. In addition, when the cells were treated with TG or LPS 6 hr after treatment with tBuBHQ, a synergistic increase on NO synthesis was shown only in the case of LPS. When phorbol 12-myristate 13-acetate (PMA), a protein kinase C (PKC) activator, was added to the cells 6 hr after the treatment with TG, there was a marked co-operative induction of NO synthesis, even though PMA alone has no effect. Based on the results obtained in this study, we suggest that cytosolic Ca2+ might be enough for the expression of iNOS gene as a priming signal and PKC might be involved in the induction of NO synthesis as a triggering signal by post-transcriptional modification of iNOS mRNA or iNOS itself in the activated murine peritoneal macrophages.

Transient thermal analysis of solidification in a centrifugal casting for composite materials containing particle segregation

Abstract: One-dimensional heat-transfer analysis during centrifugal casting of aluminum alloy and copper base metal matrix composites containing Al2O3, SiCp, and graphite particles has been studied. The model of the particle segregation is calculated by varying the volume fraction during centrifugal casting, and a finite difference technique has been adopted. The results indicate that the thickness of the region in which dispersed particles are segregated due to the centrifugal force is strongly influenced by the speed of rotation of the mold, the solidification time, and the density difference between the base alloy and the reinforcement. In the case where the base alloy density is larger than that of the particles, the thickness of the particle-rich region near the inner periphery decreases with an increase in speed, thereby increasing the volume fraction of dispersion. The solidification time of the casting is also dependent upon the speed of rotation of the mold, and it decreases with an increase in speed. This study also indicates that the presence of particles increases the solidification time of the casting.

Preparation and properties of UV-curable polyurethane acrylates

Abstract: UV-curable polyurethane (PU) acrylates have been synthesized from polypropylene glycol (PPG), isophoron diisocyanate (IPDI), and three types of reactive diluents, i.e., 2-hydroxyethylacrylate (HEA), tripropyleneglycol diacrylate (TPGDA), and trimethylolpropane triacrylate (TMPTA). The effects of soft segment length, type, and concentration of reactive diluent on the mechanical and dynamic mechanical properties have been determined. When the soft segment length was short (750) tensile strength (σb) decreased, and elongation at break (ϵb) generally increased with increasing HEA concentration, due respectively to the inferior strength of HEA homopolymer, and increased molecular weight between crosslinks (Mc). Initial modulus (E) and σb increase and elongation at break (ϵb) decreased with the increase of TPGDA concentration, and the effect was more pronounced as the soft segment length decreased. The hardness and σb increase with diluent concentration in PPG 2000-based materials was more pronounced with higher functionality diluent, due to the increased crosslinking density. The lower temperature glass transition peak of PU was not influenced by the TPGDA incorporation, whereas the higher temperature one moved toward still higher temperature. This was interpreted in terms of possible compatibility of hard segments and acrylates due to their similar polarity and hydrogen bonding. © 1996 John Wiley & Sons, Inc.

"Fermi liquid" shell model approach to composite fermion excitation spectra in fractional quantum Hall states.

Abstract: Numerical results for the energy spectra of $N$ electrons on a spherical surface are used as input data to determine the quasiparticle energies and the pairwise "Fermi liquid" interactions of composite fermion (CF) excitations in fractional quantum Hall systems. The quasiparticle energies and their interactions are then used to determine the energy spectra, $E$ vs total angular momentum $L$, of states containing more than two quasiparticles. The qualitative agreement with the numerical results gives a remarkable new confirmation of the CF picture.

Diffusive Shock Acceleration in Oblique MHD Shocks: Comparison with Monte Carlo Methods and Observations

Abstract: We report simulations of diffusive particle acceleration in oblique magnetohydrodynamical (MHD) shocks. These calculations are based on extension to oblique shocks of a numerical model for ``thermal leakage'' injection of particles at low energy into the cosmic-ray population. That technique, incorporated into a fully dynamical diffusion-convection formalism, was recently introduced for parallel shocks by Kang \& Jones (1995). Here, we have compared results of time dependent numerical simulations using our technique with Monte Carlo simulations by Ellison, Baring \& Jones 1995 and with {\it in situ} observations from the Ulysses spacecraft of oblique interplanetary shocks discussed by Baring \etal (1995). Through the success of these comparisons we have demonstrated that our {diffusion-convection} method and injection techniques provide a practical tool to capture essential physics of the injection process and particle acceleration at oblique MHD shocks. In addition to the diffusion-convection simulations, we have included time dependent two-fluid simulations for a couple of the shocks to demonstrate the basic validity of that formalism in the oblique shock context. Using simple models for the two-fluid closure parameters based on test-particle considerations, we find good agreement with the dynamical properties of the more detailed diffusion-convection results. We emphasize, however, that such two-fluid results can be sensitive to the properties of these closure parameters when the flows are not truly steady. Furthermore, we emphasize through example how the validity of the two-fluid formalism does not necessarily mean that {\it steady-state} two-fluid models provide a reliable tool for predicting the efficiency of particle acceleration in real shocks.

Catalytic skeletal isomerization of n-butenes to isobutene over natural clinoptilolite zeolite

Abstract: The proton form of a natural zeolite, clinoptilolite, was found to be an efficient catalyst for the skeletal isomerization of n-butenes to isobutene. The acidity and characteristic pore structure of this zeolite appeared to be responsible for the good performance. The effects of process variables were studied. When reaction conditions were chosen such that the dimerization reaction, the major side reaction, was suppressed, high selectivities to isobutene were obtained.

Reference values for cook's distance

Abstract: Cook's distance (Cook, 1977) is often used to assess the influence of single observations or sets of observations in the linear regression model, and it is available, at least for single observatio...